The overall goal of this proposal is to understand the mechanisms which position the spindle. In many cells, the position of the spindle determines the position of the division plane, and is thus crucial for development. The positioning of the spindle is a mechanical phenomenon and thus must be explained in terms of both forces and molecular activities, but there have been no measurements of the forces involved. We have recently developed an experimental system which will allow us to measure the relevant forces in mouse oocytes. The processes which position the spindle in mouse oocytes are unclear, but it is known that they are actin dependent. We will use magnetic tweezers to exert forces on spindles assembled in metaphase of meiosis two in mouse oocytes. We will thoroughly characterize the forces that position the spindle and use molecular perturbations, laser ablation, and rheological measurements to determine the contribution of processes proposed to contribute to spindle positioning: direct tethering of the spindle to the cortex, active force production from the cytoplasmic actin meshwork, and forces generated by actin driven cytoplasmic streaming. This work will establish the biophysical mechanisms responsible for the maintenance of the second meiotic spindle in mouse oocytes and may provide insight into spindle positioning in other systems. Understanding the mechanisms of the positioning of the second meiotic spindle may have medical relevance because improper positioning of the second meiotic spindle is a known cause of reduced fertility and is even used as a measure of oocyte quality in in-vitro fertilizatio clinics.